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Simultaneous coating of Si and B on Nb-Si-B alloys by a halide activat…

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Abstract
Phase formations during Si and B co-deposition by a halide activated pack cementation on Nb–Si–B alloys
and the oxidation behaviors of the alloys were investigated together with diffusion pathway analysis.
Coatings comprising Nb-borides and Nb-silicides were successfully fabricated on the surfaces of a-Nb,
T2 single-phase and Nbss + T2 two-phase alloys at 900 C, respectively. For the coating process, powder
mixtures with different Si to B ratios of (12.5:12.5) and (18.75:6.25) were used, respectively.
Regardless of the Si to B ratio in the powder mixture used, the coating layers were identified as
NbB2/NbSi2/Nb5Si3/substrate. The diffusion pathway followed the mass balance and showed one route
regardless of the compositions of powder mixtures used in this study. The sequential oxidation product
layers of the a-Nb and the T2 single phase alloy were identified as borosilicate/NbO2/NbB2/NbSi2/a-Nb
and (borosilicate + 3Nb2O5B2O3)/Nb2O5/NbB2/NbSi2/T2, respectively. The diffusion pathway for the oxidation
reaction was proposed using a schematic quaternary isothermal diagram.